The present invention relates to a method of analysing a gas mixture containing one or more inflammable gases to determine the explosibility of the mixture.
It also relates to an analysis system for implementing a method of this kind.
A particularly interesting application of the method and system is determining the danger of explosion of a mixture of gas and inflammable vapour in production installations liable to give off inflammable gases, in particular in the chemical, petroleum, mining and gas industries.
There are various prior art techniques for determining the risk of explosion of a gas mixture, in particular by chromatography, mass spectrometry, flame ionization and optical techniques of analysing the mixture.
One of the most widely used techniques, because it is so simple to use, employs two thermochemical elements in a Wheatstone bridge, one of which is active and the other of which has received an appropriate surface treatment to render it incapable of burning the gas mixture.
The thermochemical elements are supplied with electrical energy to heat them to the temperature of catalytic combustion of the or each inflammable gas.
Obviously, the heat of the thermochemical element depends on the heat given off during catalytic combustion and on the supply current.
By comparing the measurement signals from the Wheatstone bridge it is possible to determine the heat given off during combustion, which depends on the concentration of each inflammable gas, its calorific value and its diffusion coefficient, and the explosibility of the gas mixture. Explosibility is generally expressed as a percentage of the lower explosibility limit (LEL).
This type of technique has a number of major drawbacks, in particular because it generates additive errors due to drift with time of the Wheatstone bridge circuit and multiplicative errors due primarily to the different diffusion coefficients of the inflammable gases and their different calorific values.
These errors cause divergences in the response curves for the various gases, i.e. variations of the explosibility measurement signal of as much as 80% for different compositions of the mixtures.
Also, using two thermochemical elements which must be supplied with electrical power considerably reduces the autonomy of the measuring instruments when they are in the form of portable devices.
Attempts have been made to overcome the drawbacks by using only one thermochemical element and determining the explosibility of the gas mixture from a comparison of two measurement signals obtained during a transient phase of combustion during which the concentration of the or each inflammable gas in the gas mixture falls.
This technique considerably limits the power consumption of the measuring instrument and eliminates additive errors.
It does not eliminate multiplicative errors, however.
The object of the invention is to overcome these drawbacks.
It therefore consists in a method of analysing a gas mixture containing at least one inflammable gas to determine its explosibility, including the following steps:
energizing a resistive heating element in an analysis enclosure communicating with the gas mixture to be analysed to burn the gas mixture in the enclosure;
measuring an electrical signal at the terminals of the resistive element during combustion; and
determining the explosibility of the gas mixture from a comparison of values of the signal measured during a transient phase in which the concentration of the inflammable gas or gases in the gas mixture is falling;
characterized in that the measurements taken during the transient phase are taken at times chosen to obtain, for different inflammable gases, substantially identical measurement signals for concentrations corresponding to identical explosibilities.
The analysis method of the invention can also have one or more of the following features:
the analysis enclosure has an inlet orifice for the gas mixture calibrated to assure the diffusion of a quantity of gas towards the enclosure during the transient phase which is less than the quantity of gas burned therein;
the measurements are taken at times corresponding to values of the measurement signal from approximately 80% to approximately 40% of the maximum value of the electrical signal at the terminals of the resistive element;
if the gas mixture includes a plurality of inflammable gases having different combustion regimes a cycle of analysing said mixture is performed for each inflammable gas by energizing the resistive element to heat it to the combustion temperature of the gas, measuring the electrical signal and determining the explosibility of the gas mixture, and the result obtained is added at the end of each analysis cycle;
the analysis enclosure communicates with the gas mixture to be analysed via a porous partition.
The invention equally consists in a system for analysing a gas mixture containing at least one inflammable gas to determine its explosibility, for implementing the method defined above, characterized in that it includes an analysis enclosure in communication with the gas mixture to be analysed and having a resistive heating element disposed in the internal volume of the enclosure and a central analysis unit including means for energizing the resistive heating element to heat it to the combustion temperature of the or each inflammable gas, means for calculating the explosibility of the gas mixture including means for measuring an electrical signal at the terminals of the heating element and means for comparing values of the electrical signal at the terminals thereof, and in that the central analysis unit includes means for controlling the operation of the measuring means so as to take said measurements during a transient phase of reduction of the concentration of the or each inflammable gas in the gas mixture, at times chosen to obtain, for different inflammable gases, substantially identical measurement signals for concentrations corresponding to identical explosibilities.
The analysis enclosure advantageously has a gas mixture inlet orifice, said orifice being calibrated to assure the diffusion of a quantity of gas towards the enclosure during the transient phase less than the quantity of gas burned therein.
The analysis enclosure preferably includes a combustion chamber in which the resistive heating element is disposed and an expansion chamber communicating with the gas mixture to be analysed via a porous partition, a separator wall incorporating said calibrated orifice lying between the combustion chamber and the expansion chamber.